Resuscitation treatments for patients suffering from cardiac arrest generally include clearing and opening the patient's airway, providing rescue breathing for the patient, and applying chest compressions to provide blood flow to the victim's heart, brain and other vital organs. If the patient has a shockable heart rhythm, resuscitation also may include defibrillation therapy. The term basic life support (BLS) involves all the following elements: initial assessment; airway maintenance; expired air ventilation (rescue breathing); and chest compression. When all these elements are combined, the term cardiopulmonary resuscitation (CPR) is used.
There are many different kinds of abnormal heart rhythms, some of which can be treated by defibrillation therapy (“shockable rhythms”) and some which cannot (non-shockable rhythms”). For example, most ECG rhythms that produce significant cardiac output are considered non-shockable (examples include normal sinus rhythms, certain bradycardias, and sinus tachycardias). There are also several abnormal ECG rhythms that do not result in significant cardiac output but are still considered non-shockable, since defibrillation treatment is usually ineffective under these conditions. Examples of these non-shockable rhythms include asystole, electromechanical disassociation, and other pulseless electrical activity. Although a patient cannot remain alive with these non-viable, non-shockable rhythms, applying shocks will not help convert the rhythm. The primary examples of shockable rhythms, for which the caregiver should perform defibrillation, include ventricular fibrillation, ventricular tachycardia, and ventricular flutter.
After using a defibrillator to apply one or more shocks to a patient who has a shockable ECG rhythm, the patient may nevertheless remain unconscious, in a shockable or non-shockable, perfusing or non-perfusing rhythm. If a non-perfusing rhythm is present, the caregiver may then resort to performing CPR for a period of time in order to provide continuing blood flow and oxygen to the patient's heart, brain and other vital organs. If a shockable rhythm continues to exist or develops during the delivery of CPR, further defibrillation attempts may be undertaken following this period of cardiopulmonary resuscitation. As long as the patient remains unconscious and without effective circulation, the caregiver can alternate between use of the defibrillator (for analyzing the electrical rhythm and possibly applying a shock) and performing cardiopulmonary resuscitation (CPR). CPR generally involves a repeating pattern of five or fifteen chest compressions followed by a pause during which two rescue breaths are given.
Defibrillation can be performed using an AED. The American Heart Association, European Resuscitation Council, and other similar agencies provide protocols for the treatment of victims of cardiac arrest that include the use of AEDs. These protocols define a sequence of steps to be followed in accessing the victim's condition and determining the appropriate treatments to be delivered during resuscitation. Caregivers who may be required to use an AED are trained to follow these protocols.
Most automatic external defibrillators are actually semi-automatic external defibrillators (SAEDs), which require the caregiver to press a start or analyze button, after which the defibrillator analyzes the patient's ECG rhythm and advises the caregiver to provide a shock to the patient if the electrical rhythm is shockable. The caregiver is then responsible for pressing a control button to deliver the shock. Following shock delivery, the SAED may reanalyze the patient's ECG rhythm, automatically or manually, and advise additional shocks or instruct the caregiver to check the patient for signs of circulation (indicating that the defibrillation treatment was successful or that the rhythm is non-shockable) and to begin CPR if circulation has not been restored by the defibrillation attempts. Fully automatic external defibrillators, on the other hand, do not wait for user intervention before applying defibrillation shocks. As used below, automatic external defibrillators (AED) include semi-automatic external defibrillators (SAED).
Both types of defibrillators typically provide an auditory “stand clear” warning before beginning ECG analysis and/or the application of each shock. The caregiver is then expected to stand clear of the patient (i.e., stop any physical contact with the patient) and may be required to press a button to deliver the shock. The controls for automatic external defibrillators are typically located on a resuscitation device housing.
AEDs are typically used by trained medical or paramedic caregivers, such as physicians, nurses, emergency medical technicians, fire department personnel, and police officers. The ready availability of on-site AEDs and caregivers trained to operate them is important because a patient's chances of survival from cardiac arrest decrease by approximately 10% for each minute of delay between occurrence of the arrest and the delivery of defibrillation therapy.
Trained lay caregivers are a new group of AED operators. For example, spouses of heart attack victims may become trained as lay caregivers. Lay caregivers rarely have opportunities to defibrillate or deliver CPR, and thus they can be easily intimidated by an AED during a medical emergency. Consequently, such lay providers may be reluctant to purchase or use AEDs when needed, or might tend to wait for an ambulance to arrive rather than use an available AED, out of concern that the lay provider might do something wrong.
Some trained medical providers, e.g., specialists such as obstetricians, dermatologists, and family care practitioners, also rarely have the opportunity to perform CPR and/or defibrillate, and thus may be uneasy about doing so. Concerns about competence are exacerbated if training is infrequent, leading the caregiver to worry that he or she may not be able to remember all of the recommended resuscitation protocol steps and/or their correct sequence.
Similarly, both medical and lay caregivers may be hesitant to provide CPR and rescue breathing, or may be unsure when these steps should be performed, particularly if their training is infrequent and they rarely have the opportunity to use it.
It is well known to those skilled in the art, and has been shown in a number of studies, that CPR is a complex task with both poor initial learning as well as poor skill retention, with trainees often losing 80% of their initial skills within 6-9 months. It has thus been the object of a variety of prior art to attempt to improve on this disadvantageous condition. Aids in the performance of chest compressions are described in U.S. Pat. Nos. 4,019,501, 4,077,400, 4,095,590, 5,496,257, 6,125,299, and 6,306,107, 6,390,996. U.S. Pat. Nos. 4,588,383, 5,662,690 5,913,685, 4,863,385 describe CPR prompting systems. AEDs have always included voice prompts as well as graphical instructions on flip charts or placards since the earliest commercial versions in 1974 to provide both correct timing and sequence for the complex series of actions required of the rescuer (caregiver) as well as placement of the defibrillation electrodes. U.S. patent application Ser. No. 09/952,834 and U.S. Pat. Nos. 6,334,070 and 6,356,785 describe defibrillators with an increased level of prompting including visual prompts either in the form of graphical instructions presented on a CRT or on printed labels with backlighting or emissive indicia such as light emitting diodes. AEDs since the 1970s have used the impedance measured between the defibrillation electrodes to determine the state of the AED as well as appropriate messages to deliver to the rescuer (e.g. “Attach Electrodes” if the initial prompts on the unit have been delivered and the impedance remains greater than some specified threshold) or to determine if there is excessive patient motion (as in U.S. Pat. No. 4,610,254.) U.S. Pat. No. 5,700,281 describes a device which uses the impedance of the electrodes to determine the state of the AED for delivering messages such as “Attach Electrodes”. Enhanced prompting disclosed in these patents provides some benefit to the rescuer in improved adherence to the complex protocol required of them to successfully revive a cardiac arrest patient, but the enhanced prompting is usually not sufficient in real world situations. U.S. Pat. Nos. 5,662,690 and 6,356,785 (and the commercially available OnSite defibrillator) attempts to improve prompting by providing a rescuer-accessible “Help” key that initiates more detailed prompting in cases in which the rescuer or test subject is confused. But testing has shown that with the heightened level of anxiety that accompanies a real cardiac arrest, rescuers rarely remember to press such a Help key. Even notifying the rescuer at the beginning of the protocol to press the Help key does not help a the confused rescuer press the Help key. Furthermore, even if the Help key is pressed, it is necessary to have the rescuer work through a series of user interface interactions via a touchscreen, softkeys or other input means, for the help software to determine at which step the rescuer is in need of additional instructions. Putting the user through these interactions with the help software detracts from the rescuer's ability to provide aid to the patient, and thus delays delivery of therapy.
AEDs have also been solely focused on defibrillation, which, while it provides the best treatment for ventricular fibrillation and certain tachycardias, is of no therapeutic benefit for the 60% of the cardiac arrest patients presenting in pulseless electrical activity (PEA) or asystole. As AEDs are becoming more prevalent in the home, there are also a host of other health problems that occur such as first aid as well as incidents related to chronic conditions such as asthma, diabetes or cardiac-related conditions for which the AED is of no benefit.
It has been found in several clinical studies that supposedly trained personnel such as physicians, nurses, and paramedics trained in advanced cardiac life support (ACLS) do not provide optimal cardiac compressions, either with respect to rate or depth of compressions. This group of caregivers has also been shown in studies to provide ventilations at an excessive rate. Over-ventilation has been shown to result in excessive intra-thoracic pressure, which impedes the diastolic filling cycle and reduces blood flow and coronary and brain perfusion pressures during CPR. European Patent EP 1157717B1 and U.S. Pat. No. 6,821,254 describe systems for determining ventilation rates from an AC, small-signal transthoracic impedance (TTI) measured between the defibrillation electrode pads. A known limitation of this measurement method is the excessive noise present on the TTI signal due to a variety of noise sources, includingbody movement, capacitive fluctuations, and electrolyte charge transfer during chest compressions. Advanced caregivers also desire an accurate measure of “tidal volume”—the actual volume of gas transferred in and out of the lungs—to better assess the care provided and to make appropriate clinical decisions. Ventilation measurement based solely on TTI is, however, incapable of providing accurate measurements of tidal volume, unless an inconvenient initial calibration procedure is performed, due to the normal random inter-patient anatomical variations.